There is now substantial evidence of two anatomically and physiologically distinct pathways through primate striate and extra-striate visual cortex. These pathways have been labelled the "motion" and "color and form" pathways, suggesting a detailed knowledge of their individual functions and interactions. However, to date, there is little direct evidence either of the role these pathways play in visual perception, or of their strict independence. The studies proposed here are designed to determine the functional role of these separate pathways in primates by measuring visual loss resulting from damage to one or the other of them. The "motion" pathway will be damaged in one study by lesions of cortical area V2, which will interrupt one of the major projections to area MT, and in a later study by inactivation of MT and/or MST. The :color and form" pathway will be damaged at an early stage in its cortical path by lesions of area V2, and in a separate experiment, by lesions of area V4 itself. Lesions will be produced by localized injections of ibotenic acid into visual field specific regions determined by physiological mapping. Visual thresholds will then be tested during controlled fixation in the visual field locations that correspond to these injections. 1. The first group of studies will assess the effects of V2 lesions on visual capacities basic to both color and motion perception. Visual acuity, luminance and chromatic contrast sensitivity, color matching, and speed and direction difference thresholds will be measured. 2. A second group of studies will examine the role of visual area V4 in the processing of color information. It is expected that removal of this area should disrupt higher level but not simpler aspects of color perception. This hypothesis will be evaluated by testing both basic capacities (luminance and chromatic contrast sensitivity) and more complex abilities (color matching) within the visual field locus corresponding to the lesion. 3. A third set of studies will examine the effect of lesions of visual areas MT and MST on the perception of visual motion. Again both basic (opposite direction discrimination and velocity difference thresholds) and higher level (direction difference thresholds and manipulations of direction coherence and noise masking in dynamic dot patterns) capabilities will be tested. An additional group of studies will be conducted to compare transitory visual effects of MT and MST lesions with permanent effects.